In general, a working mechanism provided in a construction machine such as a hydraulic excavator and the like is constituted by a boom having a base end side liftably connected to a frame on a vehicle body side, an arm rotatably connected to a tip end side of the boom, a work tool such as an excavating bucket and the like rotatably connected to the tip end side of the arm, and a boom cylinder, an arm cylinder, and a work-tool cylinder for operating the boom, the arm, and the work tool.
Here, the boom constituting the working mechanism is formed of left and right side plates facing each other in a left-right direction and extending in a front-rear direction, an upper plate joined by welding to upper end sides of the left and right side plates, and a lower plate joined by welding to lower end sides of the left and right side plates. As a result, the boom is a box-shaped structural body formed of a cross section forming a square closed sectional structure, a foot-side mounting member is provided on a rear end side of this box-shaped structural body, and an arm-side mounting member is provided on a front end side. Therefore, the boom of the hydraulic excavator becomes a lengthy welded structure with the entire length of up to several meters or more and its weight also becomes large.
On the other hand, a boom in which the left and right side plates, the upper plate, and the lower plate are formed by joining a plurality of plate materials using butt welding, respectively, is proposed. This boom according to the prior art is constituted to be able to reduce a weight of the entire structure by setting a plate thickness of each plate material in accordance with required strength and by arranging the plate material with a small plate thickness within a range as wide as possible (Patent Document 1).